1. Field of the Invention
The present invention relates to a semiconductor device, and more particularly, to a resistive random access memory device.
2. Description of the Related Art
Dynamic random access memories (DRAMs) have the advantages of having high integration density and high response speed; however, have disadvantages in terms of losing stored data when power is turned off. Nonvolatile memory devices are memory devices in which the drawbacks of DRAMs do not apply, and thus, recently various nonvolatile memory devices have been proposed. Among such nonvolatile memory devices, resistive random access memories (RRAMs) have drawn attention as nonvolatile memory devices having high integration density and high response speed like DRAMs.
A conventional storage node of a RRAM has a structure in which a lower electrode, a resistance variable layer, and an upper electrode are sequentially stacked.
FIG. 1 is a cross-sectional view of a conventional storage node S1 of a conventional RRAM.
Referring to FIG. 1, the conventional storage node S1 has a structure in which a lower electrode 10, a resistance variable layer 20, and an upper electrode 30 are sequentially stacked. The lower electrode 10 and the upper electrode 30 are formed of Pt, and the resistance variable layer 20 is formed of nickel oxide (NiOx). According to a voltage that is applied between the lower electrode 10 and the upper electrode 30, a current path 1 is formed in the resistance variable layer 20 or the current path 1 is not formed in the resistance variable layer 20. When the current path 1 is formed in the resistance variable layer 20, electrical resistance in the resistance variable layer 20 is low, and this is referred to as an ON state. If the current path 1 is not formed in the resistance variable layer 20, electrical resistance in the resistance variable layer 20 is high, and this is referred to an OFF state.
The principle of generating the current path 1 in the resistance variable layer 20 will now be described in detail. When a negative voltage is applied to the resistance variable layer 20 and a positive voltage is applied to the upper electrode 30, electrons migrate into the resistance variable layer 20 from the upper electrode 30 to the lower electrode 10. These electrons have high energy, and thus, break the bonding between oxygen and nickel in the resistance variable layer 20. Thus, oxygen atoms diffuse to the lower electrode 10 resulting in the generation of oxygen vacancies in the resistance variable layer 20, and thus, the current path 1 is formed in the resistance variable layer 20 in this manner. Such diffusion of oxygen atoms is called percolation diffusion.
However, the oxygen atoms not only diffuse in the resistance variable layer 20 but also out diffuse towards at least one of the lower electrode 10 and the upper electrode 30. Therefore, an endurance of the resistance variable layer 20 and the conventional RRAM is reduced. That is, because of the out diffusion, the characteristics of the resistance variable layer 20 are easily degraded as the switching frequency increases, and in a severe case, the characteristics of the resistance variable layer 20 can be completely lost.